There are many applications where an output driver needs to drive a high capacitive load over a trace of a printed circuit board (PCB) with no resistive termination. In such a condition, signal reflection occurs and raises signal integrity issues. Because of power dissipation constraints, it is not practicable to solve the problem with a matched termination resistor.
A possible approach to improve signal integrity in such an environment would be to insert a series damping resistor at the output buffer. This approach is successful in that it improves signal integrity, but it increases signal rise and fall times because of the RC time constant. The RC time constant is given by the product of the value of the series resistor plus the output buffer impedance, on the one hand, and the sum of the capacitive load plus the distributed trace capacitance, on the other hand. The RC time constant reduces the maximum frequency of the system in which the output buffer is used.